1. Field of the Invention
The present invention relates to a liquid crystal electrooptical device using a display member of liquid crystal or the like, particularly to a constitution thereof.
2. Description of Related Art
The most general conventional display device is CRT (Cathode Ray Tube). However, in the case of CRT volume, weight and power consumption of the device are large and particularly, CRT is not suitable for a display device having a large area. Hence, in recent years, a liquid crystal electrooptical device capable of realizing light weight formation and low power consumption formation more easily than CRT attracts attention.
According to a liquid crystal electrooptical device, a difference in the dielectric constant of a liquid crystal substance between a direction in parallel with the molecular axis and a direction orthogonal thereto is utilized and polarization of light, a light transmittance amount and a scatter amount of light are controlled by which ON/OFF, that is, brightness and darkness are displayed. As material for liquid crystal, TN (Twist Nematic) liquid crystal, STN (Super Twist Nematic) liquid crystal and ferroelectric liquid crystal are generally used.
Particularly, among liquid crystal electrooptical devices, a liquid crystal electrooptical device of an active matrix type using a semiconductor device having TFT (Thin Film Transistor) on an insulating substrate of glass or the like, for example, thin film transistors for driving pixels has been developed actively.
A panel portion of a liquid crystal electrooptical device of an active matrix type is provided with a constitution in which signal lines and scanning lines are combined in a matrix on a glass substrate and TFTs are arranged at vicinities of intersecting portions. According to the constitution, a source electrode of TFT is connected to a signal line and a gate electrode is connected to a scanning line. Further, a drain electrode is connected to a pixel electrode arranged in correspondence with maintained capacitance and the liquid crystal in a pixel region. Liquid crystal is driven by being interposed between an opposed electrode and a pixel electrode. The opposed electrode is formed on an opposed substrate.
Further, a drive circuit portion for driving the signal lines and the scanning lines in the panel portion is formed by a semiconductor integrated circuit of a single crystal and is connected to the active matrix by Tape Automatic Bonding (TAB) process or Chip On Glass (COG) process.
However, a number of electrode wirings for constituting a display screen reaches as far as several hundreds. Further, a conventional peripheral drive circuit is constituted by an IC (Integrated circuit) package or a semiconductor chip and accordingly, in order to connect a terminal thereof to electrode wirings on the substrate, the wirings must be led around and an area of a peripheral drive circuit portion becomes large in comparison with a display screen. Particularly, in the case of a display device having a large area, a number of the electrode wirings is further increased and accordingly, there poses a problem in which a number of ICs is increased resulting in an increase in cost.
Further, a method of arranging a drive portion on a substrate the same as that of a panel portion has been conceived as a method of resolving the above-described problem. The drive portion in this case is formed similar to the panel portion by using thin film transistors. However, in the case where the panel portion and the drive portion are integrally formed on the same substrate, when heat or force from outside is effected, warp or the like is caused in the entire substrate and adverse influence is effected on a peripheral drive circuit installed on the same substrate by a substrate interval control member (spacer or the like). As a result, the peripheral drive circuit is not normally operated and deterioration in reliability and durability of a liquid crystal electrooptical device is caused. Further, the panel portion and the drive portion are simultaneously formed and accordingly, the yield is deteriorated particularly in the case of a display device having a large area.
Further, as other method of resolving the above-described problem, there has been conceived a method in which a drive portion is formed on other support substrate and is adhered onto a panel array substrate (substrate formed with panel portion) or a method in which after adhering a drive portion formed on other support substrate on a panel array substrate, the support substrate is removed. Further, according to the constitution provided by the method, further small size formation and light weight formation can be achieved and promotion in reliability of a display device can be achieved. Further, according to the method, a panel array substrate and a stick substrate (substrate formed with drive portion) are separately formed and therefore, excellent products and failed products can be selected by testing electric properties before the adhering operation and an excellent product of the panel array substrate and an excellent product of the stick substrate can be adhered to each other. Accordingly, in the case of a display device having a large area, the yield and the reliability of a total of an electrooptical device can significantly be promoted.
In using the above-described method, that is, the method of forming a drive portion on other support substrate and adhering it on a panel array substrate, reliability and fabrication yield of a total of an electrooptical device is controlled mainly by the following factors.
(1) Steps of fabricating a panel array substrate and fabricating a panel.
(2) steps of fabricating a stick substrate (support substrate formed with a drive portion).
(3) steps of connecting the stick substrate and the panel array substrate.
First, in the steps of fabricating the panel array substrate and the panel of (1), amorphous silicon (a—Si) is suitably used as a semiconductor material of pixel TFTs at the panel portion. That is, amorphous silicon semiconductor which is most generally used is suitably used as the semiconductor material of the panel portion since the fabrication temperature is low, the fabrication can be carried out comparatively easily by a gas phase process and the mass production performance is excellent.
Next, according to the steps of fabricating the stick substrate of (2), silicon semiconductor having crystalline performance (polysilicon) is suitably used as a semiconductor material of TFTs of the drive portion. That is, the silicon semiconductor having the crystalline performance in which physical properties of conductivity and the like are more excellent than those of amorphous silicon and high speed driving is feasible is suitably used as the semiconductor material of TFTs of the drive portion. Further, as the silicon semiconductor having the crystalline performance, there have been known polycrystal silicon, microcrystal silicon, amorphous silicon including a crystal component, semi-amorphous silicon having a state of an intermediary between the crystalline performance and the amorphous performance and so on.
As a method of providing the silicon semiconductor in a shape of a thin film having the crystalline performance, there has been known a method in which an amorphous semiconductor film is formed and thermal energy is applied for a long period of time (thermal annealing) to thereby provide the crystalline performance. However, it becomes necessary to carry out a processing for a long period of time at high temperature of 600° C. or higher as heating temperature and accordingly, there poses a problem in which the substrate is irreversibly shrunk.
When an electrooptical device is fabricated by adhering the panel array substrate formed by the amorphous silicon semiconductor as a semiconductor material of TFTs in this way, with the stick substrate formed by the silicon semiconductor having the crystalline performance as a semiconductor material of TFTs, the following problem is caused.
The panel array substrate uses the amorphous silicon semiconductor and therefore, almost no shrinkage is caused in the substrate (shrinkage of substrate), however, the stick substrate has been subjected to a thermal treatment at high temperatures to provide the silicon semiconductor having the crystalline performance and therefore, shrinkage is caused. Further, a substrate capable of withstanding the thermal treatment at high temperatures is used for the stick substrate and accordingly, the stick substrate per se is different from the panel array substrate. That is, a difference in a shrinkage width of substrate is caused between the panel array substrate and the stick substrate.
Particularly, in the case of a display device having a large area, the shape of the stick substrate is constituted by a rectangular shape having one long side (lateral length (X-direction)×vertical length (Y-direction)) and therefore, an alignment before the heating process is delicately shifted and effects a significant influence in the long side direction.
Accordingly, in the steps of connecting the panel array substrate and the stick substrate of (3), there poses a problem in which connection failure is enhanced and reliability is deteriorated by positional shift in wiring patterns caused by the difference in the shrinkage widths of the panel array substrate and the stick substrate. FIG. 8 shows a conventional example in which a positional shift in wiring patterns is caused by a shrinkage width D in X-direction of a stick substrate 803 at an end portion of the stick substrate in X-direction. Further, the positional shift in the wiring patterns is increased toward the end portion in the long side direction (X-direction) and an area of overlapping an electrode pad 808 and an electrode pad 806 is decreased.
Further, in the steps of connecting the panel array substrate and the stick substrate of (3), the wiring patterns may slightly be shifted from each other in the connecting operation and there also poses a problem in which connection failure is enhanced and reliability is deteriorated by the positional shift in the wiring patterns in this case.